The invention relates to an optimized structure of a flexible pipe reinforced with metallic armours suited to carry effluents under pressure containing H2S. The present invention is suitable for flowline type flexible pipes, i.e. flexible pipes unwound from a boat in order to be laid on the sea bottom and connected between wellheads and/or subsea gathering facilities. The invention relates more particularly to the field of deep offshore where the pipe undergoes great tensile stresses as it is laid on the bottom, but where the stresses due to the pressure are preponderant when the pipe is in service since it then lies on the sea bottom.
The current method for designing flexible pipes consists in determining the structure of a flexible pipe by combining one or more sheaths forming a seal against the gases or the liquids carried therein with armouring layers intended to withstand stresses, i.e. the inside pressure, the outside pressure, and the tensile stresses resulting from the weight of the pipe or from the inside pressure.
One can refer to documents API RP 17B (Recommended Practices) and SPEC 17J (Specification) relating to flexible pipes.
In general, a flexible pipe comprises, from the inside to the outside, a body consisting for example of an interlocked steel tape, a polymer sealing sheath, at least one pressure armouring consisting of interlocked wires spirally wound with a small pitch, at least one layer of traction armour wires spirally wound with a long pitch. This. configuration is referred to as rough bore.
A variant of a flexible pipe can comprise an internal sealing sheath, a first armouring mainly withstanding the pressure generated by the fluid in the internal sheath, generally referred to as pressure layer, possibly a second armour essentially withstanding the traction produced notably by the pressure of the fluid. This variant is referred to as smooth bore.
In another variant, some flexible pipes comprise an armouring placed above the sealing sheath, both withstanding longitudinal tensile stresses and the circumferential component due to the inside pressure of the fluid. Such a pressure armouring can comprise two layers of reverse-pitch spiral wires whose armouring angles are close to 55xc2x0. The stresses due to the inside pressure are in this case taken up by these layers.
In case of the presence of gas H2S in the effluent carried by a flexible pipe, the quality of the steels used for manufacturing all the armouring wires and the mechanical and thermal treatments applied to these wires (notably cold drawing when forming, then possibly annealing) must be selected so that these wires jointly provide the mechanical strength required during operation and laying, and corrosion resistance in the presence of H2S. Steels having relatively low mechanical qualities are therefore used, or steels of a specific composition combined with suitable thermal treatments. In the first case, this leads to disadvantageous steel weights, in the other case, it often leads to higher manufacturing costs. Documents WO-91/16,461 and WO-96/28,575 can be mentioned here by way of reference.
The present invention is based on the concept of a flexible pipe structure comprising armourings made from wires referred to as  less than  less than H2S greater than  greater than  so as to meet the criteria known in H2S ambiences, and armouring wires made of a material that does not meet H2S resistance criteria. The invention also relates to a method for designing a flexible pipe structure suited to applications in a H2S ambience, a method wherein a stress level is determined in the reinforcing wires, a level below which using H2S steel is not necessary and above which H2S steels must be used for manufacturing the reinforcing wires.
A steel known as  less than  less than H2S greater than  greater than  steel corresponds to criteria which are well known in the trade. One of these criteria is the hardness (HRC) of the steel considered: below 22 HRC, it is assumed that the steel is compatible with a H2S ambience. There is an equivalence between the hardness and the breaking strength Rm, in the present case a 22 HRC hardness corresponds to a breaking strength of about 775 to 800 MPa. This breaking strength is associated with a relatively low elastic limit of the steel, less than or equal to about 700 MPa. Another criterion allows to disregard the hardness criterion which is sufficient but not obligatorily necessary for certain steel types that have been subjected to a mechanical and/or thermal treatment. A steel is also considered to be compatible with a H2S ambience if, according to the TM0177-96 standard (method A), it can be shown that no failure appears after representative samples have been immersed for thirty days in a H2S solution and placed under a stress defined for example as equal to 90% of the elastic limit Rp0.2. This test is known as SSCC or sulfide stress corrosion cracking test. In parallel with test TM0177-96, another test known as  less than  less than HIC greater than  greater than  must be carried out. The test according to the TM0284-96 standard is relative to the blistering effects induced by the hydrogen (Hydrogen Induced Cracking) present in a type A solution, defined in the test procedure. For a steel to be compatible with H2S, only very limited damage by volume is allowed, for example of the order of 1%, preferably below 3% of the projected area. Documents WO-91/16,461 and WO-96/28,575, already mentioned above, which describe H2S steels, can be mentioned by way of reference.
The present invention thus relates to a flexible tubular pipe for carrying fluids under pressure containing H2S. The pipe comprises a body, an internal sealing sheath, at least one steel armouring layer withstanding the stresses induced by the inside/outside pressure difference, at least one traction-resisting steel armouring layer spirally wound at an angle less than 45xc2x0 in relation to the axis of the pipe. In this pipe according to the invention, at least the armouring layer withstanding the stresses induced by the side/outside pressure difference is made of a determined steel meeting H2S resistance criteria, and at least the traction-resisting armouring layer is made from wires that do not meet the H2S resistance criteria.
The pipe can comprise an intermediate sheath on the outside of which, in relation to the inside of the pipe, the traction-resisting armouring layer made from steel wires that do not meet the H2S resistance criteria is arranged.
The steel armouring layer withstanding the stresses induced by the inside/outside pressure difference can comprise at least reinforcing wires wound at an angle close to 90xc2x0 in relation to the axis of the pipe.
The steel armouring layer withstanding the stresses induced by the inside/outside pressure difference can comprise at least reinforcing wires wound at an angle close to 55xc2x0.
In the previous variant, the pipe can comprise in combination, from the inside to the outside, a body, a sealing sheath, two H2S steel armouring layers crossed at 55xc2x0, two non H2S steel armouring layers crossed at an angle less than 40xc2x0.
In another variant, the pipe according to the invention can comprise in combination, from the inside to the outside, a body, a sealing sheath, a pressure layer made of wires spirally wound at an angle close to 90xc2x0 and two armouring layers crossed at an angle less than 40xc2x0 made of H2S steel, and two armouring layers crossed at an angle less than 40xc2x0, made of non H2S steel.
The invention also relates to a method for designing a flowline type flexible pipe used for carrying fluids under pressure containing H2S once laid on the sea bottom, said pipe comprising a body, an internal sealing sheath, at least one steel armouring layer withstanding the stresses induced by the inside/outside pressure difference, at least one traction-resisting steel armouring layer spirally wound at an angle less than 45xc2x0 in relation to the axis of the pipe. The method comprises the following stages:
determining a stress level in a non H2S steel that can be used for said traction-resisting armouring layer, a level below which the steel retains, in a H2S ambience, a sufficient mechanical strength for said pipe,
dimensioning said traction-resisting steel armouring layer spirally wound at an angle less than 45xc2x0 in relation to the axis of the pipe so that the stresses induced under working pressure conditions in the wires of said traction layer are below said level,
manufacturing said traction-resisting steel armouring layer spirally wound at an angle less than 45xc2x0 in relation to the axis of the pipe from said steel that does not meet the H2S resistance criteria.